The present invention relates to a process for the chlorinating volatilization of metals which constitute impurities detrimental to iron production from a raw material which contains oxides of the impurity metals and iron and/or which forms them in an oxidizing atmosphere.
In the case of pure pyrites or pyrrhotites, which do not contain detrimental amounts of non-ferrous metals, such as copper, zinc and lead, their calcines are usable as such as raw material for the production of iron. The calcines are either pelleted, sintered or reduced as such to spongy iron. Such pyrite or pyrrhotite ores and concentrates are often impure, i.e. they contain the above-mentioned non-ferrous metals in such amounts that their calcines cannot as such be used as raw materials for iron production, but they have to be purified first in some manner. Sometimes the concentrations of these substances detrimental to iron production, especially the concentrations of noble metals, can be so high that their removal and recovery from the calcines is financially profitable, but the share of the iron ore obtained is, however, significant for the economy of the entire treatment. The methods for purifying such calcines include sulfating and chlorinating treatments, and combinations of these. Sulfating is carried out at a temperature of 650.degree.-700.degree. C. either as a direct sulfating of the concentrates or so that, for example, 2/3 of the concentrate is first dead roasted and the product of this dead roasting is then sulfated by means of the remaining 1/3 of the concentrate. During sulfating, the temperature and the oxygen pressure are so high that the iron practically remains as hematite but the copper, zinc, cobalt and nickel are converted to soluble sulfates and can be leached out and treated separately, and purple ore is obtained as raw material for iron production. One of the disadvantages is the fact that lead and noble metals are left in the purple ore.
There are several chlorination methods, but only the Kowa Seiko process has been developed to an industrial scale (U.S. Pat. No. 3,482,964). Duisburger Kupferhutten has its own chlorinating roasting process, in which sodium chloride is added in connection with sulfating, whereby soluble sulfates and chlorides are obtained. Several articles have been published on the Kowa Seiko Process, for example, Yasutake Okuho: "Kowa Seiko Pelletizing Chlorination Process--Integral Utilization of Iron Pyrites", Journal of Metals, March 1968, p. 63-67.
In the Kowa Seiko process, the pyrite or pyrrhotite calcine is finely ground, pelletized with lime and calcium chloride dried, and fed into a revolving tube furnace, in which the temperature of the pellets is increased by countercurrent heating in an oxidizing atmosphere to about 1250.degree. C., whereby the metal oxides are chlorinated by the action of calcium chloride, and the produced metal chlorides volatilize and pass into the flue gases, from which they are recovered by a hydrometallurgical treatment, and the chlorine is regenerated to calcium chloride. During the chlorination, the calcium of the calcium chloride is converted to CaO and can react with hematite to calcium ferrite, CaO.Fe.sub.2 O.sub.3, or with silicic acid to calcium silicate, CaO.SiO.sub.2. The rule is that in the final product the molar ratio of CaO to SiO.sub.2 must be 0.5-0.8, in order for the pellets to be firm enough to be fed into the blast furnace. Obviously for this reason, the total concentration of the metals to be volatilized must not exceed 2.5%. If there is not enough silicic acid, SiO.sub.2, to combine to silicates the existing CaO or the CaO produced during the chlorination, there may form CaO.Fe.sub.2 O.sub.3 with a melting point of 1216.degree. C., and the pellets disintegrate or sinter as early as during the chlorination. Considering that, during roasting, metal oxides tend to form ferrites, MeO.Fe.sub.2 O.sub.3, easily, the formation of calcium ferrites during the chlorination is highly probable. EQU CaCl.sub.2 +MeO.Fe.sub.2 O.sub.3 .fwdarw.CaO.Fe.sub.2 O.sub.3 +MeCl.sub.2 ( 1)
Calcium ferrite forming in low concentrations only keeps the pellets cohesive, but after it exceeds a certain limit it obviously weakens them. Furthermore, the process is not particularly advantageous in terms of thermal economy, for the hot product, about 900.degree.-950.degree. C., obtained from the roasting of sulfides must be cooled for grinding and pelletization, whereby the amount of heat corresponding to its enthalpy is lost. This corresponds to about 670 MJ/one tonne of pyrite. Other chlorination processes are based on the use of elemental chlorine and they are only at a pilot plant stage.
The object of the invention is to eliminate the above-mentioned disadvantages and to produce a process in which compounds with low melting points cannot form and in which substantially less chlorinating agent is used than thus far.